The present invention relates to enameled steel plates having good insulating properties and water resistance and suitable for use in thick film hybrid integrated circuits.
Conventional thick film hybrid integrated circuits are typically formed on bases having an alumina content of at least 96% by weight (hereinafter referred to as alumina bases). Although alumina bases are good in heat resistance, chemical stability, mechanical strength, and compatibility with paste raw materials, they tend to be expensive, which is especially significant with respect to thick film hybrid integrated circuits. Therefore, less expensive raw materials have been under study for many years as alternatives to alumina bases, and various materials such as steatite, forsterite, glass, enamel, anodized aluminum, synthetic resins composed of organic materials, etc. have been investigated, but they had the disadvantages that there was no highly reliable printed circuit paste suitable for them and that the fundamental properties of the base materials are poorer than those of alumina.
On the other hand, whereas ceramics and glass, including alumina, have a disadvantage of being vulnerable to mechanical impact, enameled steel plates and anodized aluminum are strong against mechanical impact, low in cost, have a wider latitude in shape, and are good in heat dissipation. However, they have the significant drawback of poor electrical insulating properties.
In the early enameled steel plate, frits for enamels containing Na.sub.2 O in an amount of 2 to 5 mol % were generally used, where a linear thermal coefficient of expansion of the frits was low as 6.times.10.sup.-6 to 7.5.times.10.sup.-6 /.degree.C. as compared with that of a steel plate of about 15.times.10.sup.-6 /.degree.C., and the frits consequently had a disadvantage of poor adherence properties with the steel plate. And in order to improve the adherence property, increasing the content of Na.sub.2 O caused the insulation to decrease.
Further, with the recent advance in automobile electronics, these hybrid integrated circuits have been increasingly used in automobiles, and in such cases, since they are typically used under environmental conditions of very high temperature and humidity (for example of a temperature of 60.degree. C. and a relative humidity (RH) of 95%) in contrast with typical environmental conditions for electronic computers, conventional enameled steel plates have revealed a problem of being inadequate in water resistance under such high temperature and high humidity conditions. The reason for that is that P.sub.2 O.sub.5 has a tendency to easily incorporate a great amount of hydroxyl group in the frit.